The present invention relates to an X-ray or positron computed tomography (CT) apparatus which is so designed that a subject to be examined can be slid into a tunnel in a gantry while he is in a sitting position.
FIGS. 6 and 7 show a recently proposed positron CT apparatus capable of high resolution. As shown, it consists generally of a gantry 1 in an annular form and a holder 3 such as a bed for fixing a subject 2. The gantry 1 consists of an annular array of sectorial radiation detecting cassettes 4a for detecting gamma-rays. Every cassette contains four units of radiation detector 4 each consisting of a scintillator 5 and a photomultiplier tube 6. These cassettes are superposed in five stacks. The radiation detectors 4 are provided with shield collimators 7 and 8. As shown specifically in FIG. 7, the shield collimator 7 is provided on the inner side of the gantry and the shield collimator 8 on its outer side so as to reduce the entry of background noise (i.e., scattered rays or a single gamma-ray) coming from areas other than the site to be examined. The radiation detectors 4 are also provided with a slice collimator 9 for each scintillator 5.
The conventional CT apparatus are intended to perform diagnosis with the subject being fixed in a supine position on a holder which is shaped like a sleeping bed. With such apparatus, it has been easy to cause the subject to slide into a tunnel in a gantry. However, as the use of high-resolution positron CT apparatus has increased these days in order to examine the head of a subject, there has arisen a growing need for examination with the subject being in a sitting position. In the conventional CT apparatus, adjustments for the straight movement and rotation of the gantry are performed independently of adjustments for the horizontal and vertical movements of the holder, and if the apparatus uses a small-diameter tunnel as in a CT apparatus designed to examine the head, it has been quite tedious and has required considerable skill to insure that the head of the subject who is in a sitting position is slid into the tunnel without bumping against its inner surface. Further, a projector for correct positioning of the head cannot be installed within the tunnel because of its small diameter.